Update the documentation for LoadCommittedDetails::previous_url.
Update the documentation for LoadCommittedDetails::previous_url.

BUG=552317
TEST=No behavior change.

Review URL: https://codereview.chromium.org/1406013009

Cr-Commit-Position: 972c6d2dc6dd5efdad1377c0d224e03eb8f276f7@{#358716}

axinging/chromium-crosswalk,axinging/chromium-crosswalk,axinging/chromium-crosswalk,axinging/chromium-crosswalk,axinging/chromium-crosswalk,axinging/chromium-crosswalk,axinging/chromium-crosswalk,axinging/chromium-crosswalk,axinging/chromium-crosswalk,axinging/chromium-crosswalk,axinging/chromium-crosswalk